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xen-devel
[Xen-devel] Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend d
Blue Swirl wrote:
On 8/4/08, Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
+/* i386 protocol version */
+#pragma pack(push, 4)
What's wrong with __attribute__(__aligned__)?
+ * FIXME: the code is designed to handle multiple outstanding
+ * requests (using aio or using threads), which isn't used right
+ * now due to limitations of the qemu block driver interface.
Yes, what happened to vectored DMA patches?
Believe it or not, they were hurting performance in KVM. Passing a
vector to the block layer will force the IO operation to be split up
into multiple requests that are all executed in serial (since QEMU only
supports 1 outstanding request at a time). Copying the vectored IO to a
linear buffer increased performance significantly because it avoided
this serialization.
The block layer needs some refactoring to support proper vectored AIO
operations before a zero-copy API will make sense from a performance
perspective.
Regards,
Anthony Liguori
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